1. Field of the Invention
The present invention relates generally to circuits, and more specifically, to voltage triggered current sink circuits where the sink current is regulated when the voltage applied across the current sink circuit exceeds a voltage threshold level.
2. Background Information
In certain applications of electronic circuits it is desirable to sink a regulated current from a power source providing a supply voltage applied across the circuit. Furthermore, in certain applications it is required to regulate this sink current only when the voltage applied across the circuit exceeds a voltage threshold level. At voltages below this voltage threshold level, the current sink circuit may be designed to conduct substantially zero current in order to reduce power consumption from the supply or as part of a classification/recognition procedure.
An example of such a classification/recognition procedure is part of the IEEE 802.3af standard. This standard describes the classification/recognition characteristics that must be displayed by electronic equipment connected to a power source that uses Ethernet cabling as a means to apply a supply voltage to the electronic equipment. In such applications, according to the IEEE 802.3af standard, as part of the operation of the electronic equipment receiving a supply voltage from the Ethernet cable, the electronic equipment must include a current sink circuit designed to sink a regulated current over a range of supply voltages applied across the current sink circuit. The current sink circuit used for this purpose should sink substantially zero current at voltages below a threshold value. The current sink circuit employed therefore must be responsive to the voltage applied across it to act as a voltage triggered current sink circuit. Known circuits that exhibit these characteristics include a voltage threshold setting element and a separate current regulation reference element.